The present invention relates to a device for detecting metallic objects in a flow of non-metallic material which comprises a scanning coil having a primary coil and a secondary coil, a current source adapted to feed at least two alternating currents with different frequencies to the primary coil, and an output circuit for the secondary coil, said output circuit delivering a signal when a metal object enters into the sensing range of the scanning coil.
Devices of the above-mentioned kind are used, among other things, for detecting metallic objects in a flow of materials such as coal, ore, logs, wood chips, wood pulp or the like non-metallic materials, the scanning coil being so designed and positioned that the flow of material moves past or, even better, through the scanning coil.
The scanning coil is suitably constructed with a primary coil and a secondary coil which is divided into two similar coils located symmetrically on opposite sides of the primary coil in the direction of material flow through the scanning coil. The secondary coils are connected such that the output signal therefrom is balanced, so that it becomes approximately zero all the while no metallic object appears within the sensing range of the coil. In this way the detector device can be designed to operate with very high sensitivity.
The problem is, however, that a consequence of high sensitivity is a greater risk of signals being generated by harmless or uninteresting objects, for example objects in the material flow having a low conductivity or larger objects of iron or metal in the vicinity of the scanning coil, but not in the material flow. As examples of objects with low conductivity may be mentioned wet logs of wood or other moist material.
An object of the present invention is to eliminate such erroneous signals without significant loss of sensitivity for the detecting of foreign matter in the material flow.